You're mistaken about your dose. It's either .0125 mgs (milligrams) which is the same as 12.5 mcgs (micrograms)... but it cannot be 12.5 mgs, which is more than enough for a herd of hypothyroid horses.

In any event, as Dawn says, everyone is different. My guess, though, is that it'll probably take you quite a while since you're starting with what is considered to be a "non"-dose. The typical starting dose is 50 - 100 micrograms. Until you work up to that, you might not notice much. I hope I'm wrong.